The periodontal ligament (PDL) is the fibrous tissue connecting the teeth and the alveolar bone and is critical for the physiological functions of teeth Periodontal diseases are the most common reasons of tooth loss. Conventional treatments including surgical or non-surgical approaches yield limited effects on regenerating pathological periodontal tissue. Recent progress in stem cell biology has led to a promising novel therapy based on the ability PDL-derived stem cells to regenerate functioning periodontium. However, the further improvement of the stem cells based therapies are greatly impeded by the poor understanding of the PDL stem cells in vivo. Although in vitro studies have shown that multipotential stem cells can be isolated from the PDL and it was speculated that the PDL stem cells are crucial for multiple processes including tissue regeneration after injury or diseases, periodontal tissue turnover and orthodontic tooth movement. However, there was virtually no information about the in vivo localization, identity or regulating niche of the PDL stem cells. In our previous study, we have identified Gli1+ cells surrounding the neurovascular bundle as the MSCs to support the incisor mesenchyme turnover and injury repair [7]. The Wnt signaling pathway plays crucial roles on activating the quiescent Gli1+ MSCs. Furthermore, we also identified Gli1+ cells in the craniofacial bone sutures as the MSCs supporting craniofacial bones turnover and injury repair. In the current research, we propose to test the hypothesis that PDL stem cells in the adult are localized near the apical region of the teeth and can be labeled with Gli1. These Gli1+ stem cells give rise to the PDL, cementum and alveolar bone. They support both natural turnover and injury repair of the PDL tissue and are regulated by Wnt signaling in vivo. This application also lays out a five-year research and training program with the ultimate goal to transition myself to an independent clinical scientist working on stem cell research. This funding will provide me with concepts, data, training and experience for future funded independent research. My mentors are leaders in the fields of craniofacial biology, stem cell biology and biological imaging. I will take advantage of the rich resources of the environment in the Center for Craniofacial Molecular Biology of University of Southern California to prepare myself for an independent research career.